There are three gears in combination in this modelâinternal, spur, and mutilated gear in center. The large or internal gear and mutilated gear are both drivers. When the teeth, on the internal gear mesh with the teeth on the small spur gear the motion is in one direction. When teeth in the mutilated gear mesh with spur gear the action is reversed. The red target disk indicates this. Motion is transmitted to the target disk by a chain drive. This movement is usable in any type of machine where advance is slow and return fast or vice versa.
[Virginia Downward & William M. Clark, 1930]

[Editorâs Note: The use of partially toothed gears may be found in the machine drawings of Leonardo da Vinci, Codex Madrid I, e.g. Folio 17 recto. FCM]